The present invention relates to a long chain N-acyl acidic amino acid or a salt thereof, and a simple process for producing the same. More specifically, the present invention relates to a long chain N-acyl acidic amino acid or a salt thereof, which has substantially no odor and can be applied even to non-perfume fields, which is diminished in a content of water soluble impurities such as inorganic salts and reaction by-products of free fatty acid, and which is suitable for the production of a detergent or a cosmetic composition, wherein the detergent prepared by incorporating it into a liquid detergent causes neither precipitation nor turbidity.
An amine or alkali metal salt of a long chain N-acyl acidic amino acid has been extensively used as a surface active agent and an antibacterial agent owing to its surface activity. Particularly, it is extensively used in detergents and cosmetic fields such as quasi-drugs and cosmetics, and in many cases comes in direct touch with the human body. Therefore, it is prohibitive to give users an unpleasant feeling. In such fields, it is frequently required that final products produce no turbidity, and the odor of the final products has an important value to the commodity. Therefore, in using the long chain N-acyl acidic amino acid or a salt thereof in such fields, it is desired to diminish impurities capable of causing turbidity of the final products and those capable of unfavorably affecting the odor of the final products to the utmost.
U.S. Pat. No. 3,758,525 discloses a process for producing a long chain N-acyl acidic amino acid, wherein an acidic amino acid and a long chain fatty acid halide are subjected to condensation reaction in the presence of an alkali using a mixed solvent of 15 to 80% by volume of a hydrophilic organic solvent and 85 to 20% by volume of water, and after the reaction is over, the reaction liquid is adjusted to pH 1, thereby precipitating a crude crystal of a long chain N-acyl acidic amino acid, which is separated by filtration and washed to remove the hydrophilic organic solvent, whereby a desired long chain N-acyl acidic amino acid is obtained. However, the long chain N-acyl acidic amino acid obtained according to said process contains inorganic salts because of insufficient removal thereof, and moreover, the process for separating the long chain N-acyl acidic amino acid as mentioned above is not industrially advantageous from a viewpoint of equipment and operation.
JP-A 51-13717 discloses a process, wherein a reaction liquid obtained by the reaction between an acidic amino acid and a long chain fatty acid halide in a mixed solvent of water and a hydrophilic organic solvent in the presence of an alkali, is adjusted to pH 1 to 6 using a mineral acid at a temperature of from 40xc2x0 C. to a boiling point of said hydrophilic organic solvent, thereby separating into an aqueous layer and an organic layer containing a desired product, and the hydrophilic solvent is then removed from the organic layer to obtain a long chain N-acyl acidic amino acid. However, according to the process, a content of inorganic salts decreases only to a degree of 1 to 2%, and odoriferous substances originating in the solvent are insufficiently removed. In Examples thereof, it is specifically disclosed that most of the acetone is removed from the organic layer by means of vacuum-heating, and then the remaining acetone is removed in a manner such that water is added to the residue and air is blown to its liquid surface while stirring the liquid at 65xc2x0 C. However, according to such a solvent removing method as blowing of air to the liquid surface, it is difficult to completely remove the remaining acetone or remove high boiling odoriferous substances mentioned below.
Further, in JP-A 3-284685 of the same applicant as that of U.S. Pat. No. 3,758,525 and JP-A 51-13717, acetone and its aldol-condensation products such as diacetone alcohol and mesityl oxide are named as substances, which remain in the long chain N-acyl acidic amino acid, and which causes an odor in the goods. And it is also disclosed therein that even when the process disclosed in JP-A 51-13717 is used, these odoriferous substances cannot be removed completely and as a result, these are left in the long chain N-acyl acidic amino acid and cause an odor of the products. On such a premise, it is further disclosed to remove these odoriferous substances and salts from an aqueous solution of a salt of the long chain N-acyl acidic amino acid by means of reverse osmosis membrane. However, the process is disadvantageous from a viewpoint of using an expensive membrane separation apparatus, and it cannot be said that the process is simple from an industrial point of view, because the process cannot be carried out without complicated operation control such as control of concentrations and control of membranes.
JP-A 50-5305 discloses that in subjecting an amino acid and a long chain fatty acid halide to condensation in the presence of an alkali, an aqueous lower alcohol is used as a reaction solvent, and as the aqueous lower alcohol, methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol and sec-butanol are specifically enumerated in a limited manner. However, all the above-mentioned alcohol are primary or secondary, and therefore in a step of making a pH of the liquid acidic, a dehydration-condensation reaction occurs between the formed long chain N-acyl acidic amino acid and said alcohol solvent, thereby resulting in the production of an ester. In addition, a dehydration-condensation reaction occurs also between said alcohol solvent and a free fatty acid by product through hydrolysis of the raw material of long chain fatty acid halide, thereby producing an ester. The thus produced ester is a compound which is difficult to separate and remove from the long chain N-acyl acidic amino acid.
JP-A 7-2747 also proposes a separation process using a membrane, which is, however, disadvantageous like in the process disclosed in JP-A 3-284685. In Comparative Example of JP-A 7-2747, it is disclosed to directly condense the obtained long chain N-acyl acidic amino acid-containing organic layer, and it is demonstrated that a free fatty acid remarkably increases in the course of said condensing, and the hydrophilic organic solvent is hardly removed.
JP-A 3-279354 discloses a reaction process wherein a mixed solvent of water and an hydrophilic organic solvent consisting of acetone and isopropanol is used to prevent the production of odoriferous components such as diacetone alcohol and mesityl oxide, which are greatly produced when acetone is used singly as the solvent. There is also disclosed a separation process wherein the acidified reaction liquid is subjected to crystallization separation to obtain crystals, which are then dissolved in a hydrophilic organic solvent, and an aqueous solution of a high concentration of sodium sulfate is added thereto, thereby separating into an organic layer and an aqueous layer. However, according to the process, there are left problems such that a step of re-dissolving the crystals once separated by crystallization is troublesome, it is inevitable for the goods to be contaminated with sodium sulfate as far as a large amount of sodium sulfate is used, and it is inevitable to treat a waste liquid containing a high concentration of sodium sulfate. Moreover, even when the mixed solvent of acetone and isopropanol is used as the hydrophilic organic solvent, it is not always sufficient to diminish diacetone and mesityl oxide to a degree such that any additional removal is not required, and as a result, it is still essential to remove these odoriferous components. In addition, with respect to removal of the organic solvent from the organic layer, an Example of said JP-A describes nothing but removal thereof by means of vacuum-heating as a specific removal means, and only describes a content of the acetone-condensation product in the long chain N-acyl acidic amino acid is a trace. It is not clear whether or not it is removed to a degree such that it does not affect the odor of the final product.
As mentioned above, a long chain N-acyl acidic amino acid, which has substantially no odor and which is diminished in a content of water soluble impurities such as inorganic salts and free fatty acids is unknown, and a simple process for producing the same is also unknown.
As a result, any long chain N-acyl acidic amino acid or its salt obtained according to a conventional process always has an odor owing to by-products originating from the reaction solvents, and contains impurities such as inorganic salts and free fatty acids, and therefore there are left problems such that it cannot be applied to a non-perfume system, and when it is incorporated into goods such as a detergent, turbidity or precipitation occurs when stored particularly at a low temperature.
Under these circumstances, it is an object of the present invention to provide a long chain N-acyl acidic amino acid, which has no effect on perfume of goods, and which has superior stability at low temperature, and it is another object of the present invention to provide a process for producing the long chain N-acyl acidic amino acid.
Generally speaking, using a mixed solvent of a hydrophilic organic solvent and water as a reaction solvent, an acidic amino acid and a long chain fatty acid halide are subjected to condensation in the presence of an alkali to produce a long chain N-acyl acidic amino acid (acylation reaction), and the thus obtained reaction liquid is adjusted to pH 1 to 6 to separate into an organic layer and an aqueous layer, thereby obtaining a long chain N-acyl acidic amino acid-containing organic layer (acid-precipitation separation step). However, the obtained long chain N-acyl acidic amino acid is insufficient in removal of inorganic salts.
The present inventors have undertaken extensive studies to solve the problems of the prior art mentioned above, and as a result, it has been found that a mixed liquid of a long chain N-acyl acidic amino acid and a medium containing at least tertiary butanol and water can be separated into an aqueous layer and a long chain N-acyl acidic amino acid-containing organic layer according to the composition of said three components, and thereby inorganic salts remaining in the long chain N-acyl acidic amino acid can be conveyed into the aqueous layer to be removed (hereinafter the operation being referred to as washing). Thus, tertiary butanol and water are added to the long chain N-acyl acidic amino acid containing inorganic salts to form the three-component system, a composition thereof is appropriately selected, and a separation-removal treatment is repeatedly carried out, whereby a desired content of inorganic salts can be attained.
The present inventors have further found a fact that said tertiary butanol used in the above-mentioned washing step can be advantageously used as a reaction solvent in the acylation reaction step for the production of long chain N-acyl acidic amino acid. In other words, it has been found that when the acylation reaction is carried out using a mixed solvent of tertiary butanol/water, there is observed no production of odoriferous substances such as aldol-condensation products, which are produced when a mixed solvent of acetone/water is used as the reaction solvent as seen in the prior art.
In the case where the obtained long chain N-acyl acidic amino acid is applied to a surface active agent, it is desired to remove the organic solvent used for the production of long chain N-acyl acidic amino acid and impurities originated from the organic solvent as far as possible. However, there is a substantial trace quantity thereof remaining. In practice, diacetone alcohol and mesityl oxide which seem to originate from the acetone solvent can be detected in a now commercially available long chain N-acyl acidic amino acid or its salt. As mentioned above, the diacetone alcohol and mesityl oxide even in a trace quantity causes a bad odor. In addition, even when these odoriferous substances could be removed as far as possible, in the resulting long chain N-acyl acidic amino acid or a salt thereof, an odor such as an odor of fatty acids still remains, and therefore it is difficult to incorporate into non-perfume cosmetics or the like.
When tertiary butanol is used as the reaction solvent, any aldol-condensation products produced when acetone is used are not produced, and therefore, it is permitted to consider the tertiary butanol itself only as the odoriferous substance remaining in products. An odor threshold of tertiary butanol is far higher in comparison with that of the acetone-condensation products such as diacetone alcohol and mesityl oxide, and therefore from a viewpoint of controlling odor, it can be said that the burden of removal is far less when comparing tertiary butanol with acetone.
Turbidity and precipitation caused when the long chain N-acyl acidic amino acid or its salt is incorporated into a liquid detergent or the like, and particularly when the incorporated composition liquid is allowed to stand at a low temperature such as about 5xc2x0 C., are mainly caused by free fatty acids and inorganic salts contained in the long chain N-acyl acidic amino acid, which are conveyed from the starting materials or produced in the course of the production of long chain N-acyl acidic amino acid. Particularly, the free fatty acids can be obtained by decomposition of the long chain N-acyl acidic amino acid, and when once obtained, the free fatty acids can hardly be separated from the long chain N-acyl acidic amino acid or its salt, and therefore it is important to prevent the yield thereof in the production step. In the production step of the long chain N-acyl acidic amino acid, an increase of the free fatty acids can be observed in a step including a thermal history such as removal of a hydrophilic organic solvent by directly condensing a long chain N-acyl acidic amino acid-containing organic layer, as disclosed in the prior art.
This is because a state of the liquid at the time when the solvent is distillation-removed from the long chain N-acyl acidic amino acid-containing organic layer is bad, in other words, flowability of the liquid is bad, and moreover the liquid is bubbling and in a very unstable state.
Usually, in the case where the long chain N-acyl acidic amino acid is separated from the mixed liquid by distillation-removal of the hydrophilic organic solvent contained in the mixed solvent of water and the hydrophilic organic solvent, it is usual that the distillation is carried out under reduced pressure from a viewpoint of heat supply. However, when the distillation-removal of the hydrophilic organic solvent from the mixed liquid is carried out under reduced pressure, the liquid usually increases its viscosity, and results in a paste having almost no flowability. Here, it has been found that the distillation-removal of the organic substances under such conditions is markedly inferior in efficiency, and almost no odoriferous substance such as acetone and acetone-condensation products including diacetone alcohol and mesityl oxide can be removed.
As mentioned above, when the distillation-removal of the hydrophilic organic solvent contained in the mixed solvent of water and the hydrophilic organic solvent is continued to separate the long chain N-acyl acidic amino acid from the mixed liquid, the liquid increases its concentration and results in a high viscosity. In order to continue the distillation while keeping the flowability of the liquid, it is necessary to raise a temperature of the liquid. Further, to continue the distillation, it sometimes happens that many dispersed bubbles are produced in the liquid. In other words, a bubbling state occurs to make the system very unstable. In such a case, it is necessary to carry out the distillation taking a great amount of time. For example, the bubbling state is controlled in an intermittent manner such that pressure of the system is increased or decreased to prohibit bumping, or the vapor quantity generated is drastically lowered.
Accordingly, in the case where the hydrophilic organic solvent is condensed to be removed in such a manner, the long chain N-acyl acidic amino acid is greatly subjected to thermal history and then decomposes to produce the decomposition products of free fatty acids. When such a salt of the long chain N-acyl acidic amino acid is incorporated into a liquid detergent, because of the increased free fatty acids in the long chain N-acyl acidic amino acid, a cosmetic composition incorporated therewith produces turbidity at a low temperature, and thereby the property essential to the product is markedly impaired.
The present inventors have undertaken extensive studies to attain removal of the hydrophilic organic solvent while preventing the free fatty acid from producing. As a result, it has been found that when the conditions such as a composition of the liquid and a temperature thereof are controlled at the time of removing the hydrophilic organic solvent from a long chain N-acyl acidic amino acid-containing mixed solution of water and the hydrophilic organic solvent, flowability of the liquid in distillation-removal of the solvent can be greatly improved, and viscosity of the liquid during the distillation operation can be kept within a favorable range even while keeping a temperature of the liquid low, and thereby the distillation-removal can be attained to a degree so as to have no effect on an odor of the goods. That is, it is a finding that in removing the hydrophilic organic solvent from the long chain N-acyl acidic amino acid-containing organic layer, the long chain N-acyl acidic amino acid is converted in its alkali salt, and either a solid concentration of the liquid during distillation is held within a fixed range under a fixed temperature condition, or a ratio between the long chain N-acyl acidic amino acid and water in the mixed liquid is maintained within a fixed range under a fixed temperature condition, provided that a composition of the organic solvent in the mixed liquid is not more than 5% by weight.
It is another finding that when the hydrophilic organic solvent is distillation-removed under the above-mentioned conditions, flowability of the liquid can be improved, whereby the liquid temperature of distillation is lowered, a thermal history can be greatly avoided, and production of the free fatty acids owing to decomposition of the long chain N-acyl acidic amino acid can be substantially prohibited. It is a further finding that the thus obtained long chain N-acyl acidic amino acid having a content of the free fatty acid limited to a fixed level can exhibit a markedly superior performance. Thereby, the present invention has been obtained.
That is, the present invention is as follows.
A process for producing a long chain N-acyl acidic amino acid, characterized by comprising a step (washing step) of removing impurities mentioned below by separating a mixture composed of a long chain N-acyl acidic amino acid containing an inorganic salt and a medium consisting essentially of water and tertiary butanol into an aqueous layer and an organic layer containing the long chain N-acyl acidic amino acid at a temperature of from 35 to 80xc2x0 C.
The above-mentioned process for producing a long chain N-acyl acidic amino acid, wherein the above-mentioned long chain N-acyl acidic amino acid is obtained through the following steps:
1) a step (acylation reaction step) of subjecting an acidic amino acid and a long chain fatty acid halide to condensation in a mixed solvent consisting essentially of water and tertiary butanol in the presence of an alkali, and
2) a step (acid-precipitation separation step) of adjusting the pH of the obtained reaction liquid to from 1 to 6 with use of a mineral acid to separate into an organic layer and an aqueous layer, thereby obtaining an organic layer containing the long chain N-acyl acidic amino acid.
The above-mentioned process, wherein the organic layer containing a long chain N-acyl acidic amino acid obtained in the above-mentioned washing step is subjected to removal of an organic solvent by distillation, in which not less than {fraction (1/20)} of carboxyl group of the long chain N-acyl acidic amino acid is converted into its alkali salt, and the distillation is carried out under conditions that a temperature of the resulting mixed liquid is controlled so not to exceed 90xc2x0 C., and water is added to maintain a solid concentration of the mixed liquid to from 5 to 50% by weight.
The above-mentioned process, wherein the organic layer containing a long chain N-acyl acidic amino acid obtained in the above-mentioned washing step is subjected to removal of an organic solvent by distillation, which is carried out under conditions that a temperature of the mixed liquid is controlled not to exceed 90xc2x0 C., and water is added to maintain a weight ratio between the long chain N-acyl acidic amino acid and water within a range of from 35/65 to 65/35, provided that a content of the organic solvent in the mixed liquid is not more than 5% by weight.
Further, the present invention provides a long chain N-acyl acidic amino acid or a salt thereof having a content of an inorganic salt of not more than 1% by weight, a content of tertiary butanol of from 0.1 to 750 ppm by weight, and/or a content of a free fatty acid of not more than 3.0% by weight, said contents being based on the weight of the long chain N-acyl acidic amino acid. Still further, the present invention provides a detergent or cosmetic composition incorporated with the long chain N-acyl acidic amino acid.